This application is based on Japanese Patent Application No. 2000-315191, filed in Japan on Oct. 16, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to a switching device which employs electromagnetic repulsion to generate a drive force to produce relative movement of a pair of contacts into or out of contact with each to close or open an electric circuit.
2. Description of the Related Art
FIGS. 10a and 10b are schematic cutaway elevations of a switching device known to the inventors which utilizes electromagnetic repulsive force in a closed contact state and an open contact state, respectively. The illustrated switching device includes a switch portion 3 which can open and close an electric circuit, a movable shaft 5 which transmits a drive force to the switch portion 3, and an operating mechanism 9 which is driven by an unillustrated electric power supply and applies a drive force to the movable shaft 5 to open and close the switch portion 3.
The switch portion 3 includes a fixed contact 1 which is secured to a support plate 16 and a movable contact 2 which is disposed opposite the fixed contact 1. In order to obtain good arc extinguishing properties for the switch portion 3, the contacts 1 and 2 are housed in an evacuated chamber 4. A first terminal 14 is electrically connected to the fixed contact 1, and a second terminal 15 is electrically connected to the movable contact 2. The switch portion 3 can be electrically connected to an external electric circuit through these terminals 14 and 15.
The movable shaft 5 includes a live portion 6 connected to the movable contact 2 and a non-live portion 7 connected to the operating mechanism 9. The live portion 6 and the non-live portion 7 are connected to and electrically insulated from each other by an electrically insulating rod 8 which prevents current from flowing from the switch portion 3 to the operating mechanism 9.
The operating mechanism 9 includes a contact opening fixed coil 11 which is secured to a stationary support plate 17, a contact closing fixed coil 12 which is secured to another stationary support plate 18, a movable coil 10 which is secured to the movable shaft 5 and which is disposed between the contact opening fixed coil 11 and the contact closing fixed coil 12, and a bidirectional biasing spring 13 which is secured to a support plate 19 and to the non-live portion 7 of the movable shaft 5. The movable shaft 5 loosely passes through support plate 17 and support plate 18, so the movable coil 10 can reciprocate between the contact opening fixed coil 11 and the contact closing fixed coil 12. The biasing spring 13 is a non-linear spring which exerts a biasing force which changes in direction depending on the position of the movable shaft 5. When the movable shaft 5 is in the raised position shown in FIG. 10a, the biasing spring 13 exerts an upwards biasing force on the movable shaft 5 to maintain the contacts of the switch portion 3 in a closed state, and when the movable shaft 5 is in the lowered position shown in FIG. 10b, the biasing spring 13 exerts a downwards biasing force on the movable shaft 5 to maintain the contacts of the switch portion 3 in an open state.
Next, contact opening operation will be explained. When the switching device is in the closed contact state shown in FIG. 10a, if a pulse current from the unillustrated power supply is supplied to the contact opening fixed coil 11 and the movable coil 10, these coils 11 and 10 generate magnetic fields which produce electromagnetic repulsive forces which repel the coils 11 and 10 from each other. The movable coil 10 is pushed downwards in the figure by the electromagnetic repulsive forces, and the movable shaft 5 which is secured to the movable coil 10 and the movable contact 2 which is connected to the movable shaft 5 also move downwards, causing the movable contact 2 to separate from the fixed contact 1, and contact opening of the switch portion 3 takes place. When the movable shaft 5 moves downwards past a prescribed point, the direction in which the biasing spring 13 exerts a biasing force on the movable shaft 5 changes from the contact closing direction (upwards in the figure) to the contact opening direction (downwards in the figure), so when the contacts 1 and 2 of the switch portion 3 are separated from each other, the biasing spring 13 maintains the switch portion 3 in an open contact state as shown in FIG. 10b. 
Next, contact closing operation will be explained. When the switching device is in the open contact state shown in FIG. 10b, if a pulse current from the power supply is supplied to the contact closing fixed coil 12 and the movable coil 10, magnetic fields are generated by these coils 12 and 10, and the magnetic fields produce electromagnetic repulsive forces which repel coils 12 and 10 from each other. The movable coil 10 is pushed upwards in the figure by the electromagnetic repulsive forces, the movable shaft 5 and the movable contact 2 move upwards with the movable coil 10, and the movable contact 2 contacts the fixed contact 1 to perform contact closing of the switch portion 3. When the movable shaft 5 moves upwards past a prescribed point, the direction in which the biasing spring 13 exerts a biasing force on the movable shaft 5 changes from the contact opening direction (downwards in the figure) back to the contact closing direction (upwards in the figure), so when the contacts 1 and 2 of the switch portion 3 are in contact with each other, the biasing spring 13 maintains the switch portion 3 in the closed contact state shown in FIG. 10a. 
In the switching device of FIGS. 10a and 10b, contact opening and closing operations are carried out by electromagnetic repulsion between opposing coils, so the speed of operation is high. As a result of the collision through magnetic force between opposing coils occurring during this high speed operation, large impacts are applied to the coils, and the coils can be damaged by these impacts.
Since the movable coil 10 is flat, it is subjected to a large bending moment near its longitudinal axis. If the thickness of the movable coil is increased in order to increase its stiffness and its resistance to impacts, the center-to-center distance between opposing coils (the distance between two opposing coils measured from halfway through the thickness of one coil to halfway through the thickness of the opposing coil) increases, and electromagnetic repulsive forces cannot be efficiently generated. Furthermore, increasing the thickness of the movable coil increases the overall size of the switching device in the axial direction, making the switching device more cumbersome.
An object of the present invention is to provide a switching device which prevents damage to opposing coils of the switching device due to impacts during contact opening or contact closing operation.
Another object of the present invention is to provide a switching device having coils which can efficiently generate electromagnetic repulsive forces.
Yet another object of the present invention is to provide a switching device which is highly reliable and has good high speed responsiveness.
According to one form of the present invention, a switching device includes a switch portion having a fixed contact and a movable contact, a movable shaft drivingly connected to the movable contact, and an operating mechanism drivingly connected to the movable shaft and moving the movable shaft to open and close the switch portion. The operating mechanism includes a flat movable coil operatively connected to the movable shaft, a fixed coil opposing the movable coil, and a coil stiffener which increases the stiffness of the movable coil against forces in the axial direction of the movable shaft.
In preferred embodiments, the movable coil has an outer diameter which is approximately 9-11 times its thickness.
The coil stiffener may have a variety of configurations. In one form of the invention, the coil stiffener comprises a resin molded around the movable coil. In another form of the invention, the coil stiffener comprises a varnish applied to the movable coil.
The coil stiffener may include a case which houses the movable coil. In preferred embodiments, the case comprises a nonmagnetic metal.
The case may include radially-extending slits or grooves in a surface thereof which opposes a fixed coil to reduce the generation of eddy currents in the case.
An electrically insulating material may be provided between the case and the movable coil to enhance insulating properties.
A ferromagnetic core may be disposed in the case in a location surrounded by the movable coil to increase the magnetic field generated by the movable coil.
The case may include a hub at a radially inner portion thereof to increase the bending stiffness of the case. In a preferred embodiment, the case includes a plurality of projections extending radially from the hub, with each projection extending into a ferromagnetic core. An electrically insulating material may be disposed between the hub, the projections, and the core.
In a preferred embodiment, the thickness of the case in its axial direction is greater at a radially inner portion thereof than at a radially outer portion thereof.
In another preferred embodiment, the case has a thickness on a side thereof which opposes a fixed coil which is smaller than a thickness on the opposite side of the case.